1. Technical Field
The present invention relates to a laser diode and a method of manufacturing the same.
2. Related Art
It has been known that laser diodes making use of Group-III nitride semiconductor materials are less likely to give a good Gaussian profile of far-field pattern. More specifically, typically as illustrated in FIG. 13, the profile of the far-field pattern occasionally contains ripples or deformation of overall shape. For reference, FIG. 12 comparatively illustrates a good Gaussian profile of the far-field pattern without ripples.
One possible reason why the irregularity occurs in the profile of the far-field pattern as illustrated in FIG. 13 resides in that the gallium nitride (GaN) or sapphire, which is used as a material for composing the substrate, is transparent to the light at a wavelength of oscillation of the semiconductor laser (405 nm for Blu-Ray Disk applications).
In general, an optical waveguide of the laser diode does not show completely 100% optical confinement, but allows leakage of light from the optical waveguide little by little. In other words, stray light produces. It is also general that the optical waveguide is not spatially uniform, but contains fluctuations in the geometry, composition of the material, crystallinity and so forth. Also the spatial fluctuation is causative of the leakage of light.
Thus-produced stray light randomly propagates through the element, irrespective of the mode of the optical waveguide. In general, the stray light may be absorbed by the substrate material, if the substrate is based on a material system other than Group-III nitride semiconductors. In this case, the stray light may be absorbed mainly by the substrate material, and thereby the stray light may immediately be attenuated, without severely affecting operations of the element. On the other hand, for the case of using the Group-III nitride semiconductors, GaN and sapphire used for composing the substrate are transparent to the light at the wavelength of oscillation. The stray light is, therefore, less likely to be attenuated through absorption by the substrate. In particular for the case of using a GaN substrate, the refractive index of the GaN substrate is often larger than the effective refractive index characterizing the waveguide mode, so that generation of the stray light and propagation thereof in the GaN substrate may be more likely to be promoted. A part of the stray light is emitted from the output end of the laser diode, at a small intensity. The stray light therefore interferes with the intrinsic laser light, and consequently disturbs the far-field pattern as illustrated in FIG. 13.
There have already been proposed several methods of improving the disturbance in the far-field pattern. Japanese Laid-Open Patent Publication No. 2005-311308 discloses a system of providing trenches on both sides of an optical waveguide, in the vicinity of the output end of the laser diode. This publication further mentions a method of providing trenches on both sides of the optical waveguide also in the vicinity of the reflective end. An effect of the trenches described in this publication is to reflect or scatter the stray light possibly emitted from around a emission spot towards the outside of the laser diode. Components of the stray light most likely to interfere with the intrinsic output light of the laser element are those emitted from around the emission spot in the nearly same direction with the intrinsic output light of laser element. It may therefore be said that this technique improves the far-field pattern, by efficiently reflecting or scattering the components of the stray light most causative of disturbance in the far-field pattern.
Other documents of the prior art relevant to the present invention include Japanese Laid-Open Patent Publication Nos. 2002-324947 and 2006-165407.
Japanese Laid-Open Patent Publication 2002-324947 discloses a laser diode having a recess in the vicinity of the output end, wherein the recess is located so as to be brought into contact with, or located in proximity to a ridge portion. Also in this case, the profile of the far-field pattern is improved by the recess.
Also in Japanese Laid-Open Patent Publication No. 2006-165407, it is described that the ripples in the far-field pattern may be suppressed by similarly providing a plurality of recesses on the oscillation surface on the light output side.
However, the techniques disclosed in each of Japanese Laid-Open Patent Publication Nos. 2005-311308, 2002-324947 and 2006-165407 respectively have problems to be solved as explained below.
The structure disclosed in Japanese Laid-Open Patent Publication No. 2005-311308 tends to make the process margin in the manufacturing more stringent. In particular, it is difficult to ensure a sufficient process margin as for the distance between each of the trenches and the optical waveguide, and the distance between each of the trenches and the end face of the element (site of cleavage).
The distance between each of the trenches and the optical waveguide will be explained first. If the trenches and the optical waveguide are too close to each other, the optical waveguide may be damaged in the process of forming the trenches by dry etching, and the element characteristics may consequently be degraded. Not only the stray light, but also the light output of the laser diode per se may partially be scattered by the trenches. If a part of the waveguide mode is disturbed by scattering, degradation in the far-field pattern may be anticipated, raising an effect against expectations. On the other hand, if the trenches and the optical waveguide are placed too far, the effect of scattering of the stray light may be weaker. It is therefore necessary to design the laser element while taking not only the optimum distance between the each of the trenches and the optical waveguide, but also process accuracy and reproducibility into consideration. In particular, mask alignment in the individual processes needs a high level of preciseness.
Next, the distance between each of the trenches and the end face of the element will be discussed. Too small distance may make cleavage of the wafer difficult. In the process of manufacturing the laser diode, the wafer may necessarily be cleaved into bars, except for special cases such as manufacturing those having surface emission structures. The Group-III nitride semiconductors are, however, difficult to be straightly cleaved as compared with other compound semiconductor materials, and are therefore difficult to yield flat cleavage surface. If the structures such as trenches reside in the close vicinity of the site of cleavage, the cleavage surface may incline towards the trenches. If the laser light is output from the inclined cleavage surface, the laser light is distorted, raising another cause for degradation in the far-field pattern. In addition, the effect of scattering the stray light by the trenches may degrade.
In view of solving these problems, Japanese Laid-Open Patent Publication No. 2005-311308 mentions also a system having the trenches disposed so as to overlap the surface of the oscillator, to thereby allow the trenches also to assist the cleavage (a system of providing cleavage-assisting trenches). However, the provision of the trenches so as to overlap the surface of oscillation allows scattering of light only once on the side faces of the trenches, only to give a limited effect of scattering. It may therefore be necessary to additionally provide trenches for scattering, besides the cleavage-assisting trenches, away from the surface of oscillation. In this case, the cleavage may occur in an inclined manner towards the trenches for scattering other than the cleavage-assisting trenches.
As is clear from the above, the technique described in Japanese Laid-Open Patent Publication No. 2005-311308 may tend to be tightly limited both from the viewpoints of the degree of freedom of design and accuracy in the manufacturing processes.
Japanese Laid-Open Patent Publication No. 2005-311308 also describes that the stray light occurs over the entire range of the longitudinal direction of the optical waveguide. According to the publication, the stray light is intercepted by the end of the oscillator of the laser diode. The present inventors, however, found out from our research and development that excellence of flatness of crystal growth layer affects deformation and ripples in the far-field pattern. The present inventors compared the elements giving good far field patterns and those giving poor far field patterns, using an instrument capable of evaluating surface irregularity on the order of nanometer, such as atomic force microscope, and found out that the elements giving good far-field patterns tend to have small surface irregularities. The surface profile of the wafer may basically be ascribable to irregularity of the crystal growth layer. Since the optical waveguide structure of the semiconductor laser is fabricated in the crystal growth layer, so that the profile of the optical waveguide inevitably contains the irregularity. From our findings, it is supposed that the irregularity in the optical waveguide may yield something like a local emission source of the stray light, and may govern degradation in the far-field pattern.
A most fundamental solution for improvement in the far-field pattern is considered to improve the flatness of the crystal growth layer. It is, however, difficult to manufacture a flat layer on the nanometer level, over the entire range of the wafer in the process of crystal growth, as far as the nitride-base semiconductor materials are used.
Same problems reside in the techniques described in Japanese Laid-Open Patent Publications Nos. 2002-324947 and 2006-165407.
In short, it has been difficult to ensure a large process margin in the manufacturing, and to improve the profile of the far-field pattern at the same time.